Derek A. Paice in his U.S. Pat. No. 5,124,904, expressly incorporated by reference herein, was concerned with AC to DC converters, such as can be used for AC or DC motors. Paice recognized one problem with static AC to DC converters, namely, the generation of harmonics on the AC lines, which are caused by the rectification process. Paice's object was to design an 18-pulse converter system with “optimized” structural and functional characteristics. The Paice invention used three 6-pulse rectifier bridges directly connected in parallel with the DC terminals, two of the bridges operating under respective +40° and −40° phase shift relative to the AC lines, the third (central) bridge being directly connected to the AC lines, and the rectifiers being controlled for 40-degree current condition.
Peter W. Hammond in his U.S. Pat. No. 5,619,407, expressly incorporated by reference herein, was concerned with autotransformers for converting 3-phase power to 9-phase power, and particularly to autotransformers for providing reduced harmonics and ripple in an AC to DC rectifier. Hammond recognized an advantage of the Paice system, referred to above, of an autotransformer passing the three input phases directly through to become three of the output phases of the autotransformer, the autotransformer generating six additional output phases to achieve a total of nine output phases. Hammond states that when the Paice system feeds an 18 pulse rectifier, the DC output voltage is about 14% higher than would result from the original 3-phase AC input power feeding a 6-pulse rectifier. Hammond, therefore, deemed it desirable to reduce the magnitude of the 9-phase AC voltage to compensate for this 14% increase in DC voltage. According to Hammond, the magnitude of voltage should be reduced for use with an existing system that previously utilized 6-pulse rectifiers so that the DC voltage remains unchanged. The Hammond invention involved an autotransformer including three coils each having a plurality of serial windings and a plurality of stub windings. The serial windings form a delta and the stub windings are magnetically coupled with serial windings from the same coil. The stub windings are additionally electrically connected at a first end thereof with respective ones of the serial windings. Three terminals are provided at the apices of the delta.
Both the Paice patent and the Hammond patent discuss earlier approaches for AC to DC power converters.